MUCH better photo. Thanks.
Treat immediately with Cupramine.
Under the (visual) circumstances, if the fish is laboring to breathe, I would perform a FW dip prior to the Cupramine treatment. Although FW won't kill off the disease, it may provide some breathing relief.
Yes. The Dwarf Angelfishes (in particular) cannot be treated with general copper mediations, EXCEPT for Cupramine. Few fishes come close to their sensitivity to copper.
From your description. . .You don't have a QT ready. All you need to do is have a sponge filter in your main display to be seeded with bacteria and then, setup any appropriately sized tank within a few minutes using that filter to have a 'live' QT.